The mechanism, by which DNA is replicated past a non-coding lesion in parent strand DNA in mammalian cells, is poorly understood. Unlike bacteria, general recombination-like events have not been convincingly seen in higher eukaryotes after DNA damage by an agent such as UV-radiation (UV). In this study, I have demonstrated a low level of recombination events as measured by the exchange of pyrimidine dimers from parent to daughter strand DNA. In order to demonstrate clearly such recombination events, cells were irradiated in the G0 or G1 phases of the cell cycle and then allowed to enter DNA synthesis. Pyrimidine dimers were measured by a very sensitive enzymatic assay using alkaline elution which I recently developed. Human peripheral blood lymphocytes (PBL), normal human (NF) and Group A xeroderma pigmentosum (XP) fibroblasts, and 3T3 cells were irradiated and then stimulated to enter DNA synthesis by addition of phytohemagglutinin or release from contact inhibition. [14C]-tdr was added to cells 20h after UV and the cells were analyzed 35h after UV. The frequency of dimers in daughter strand DNA increased with UV dose in all cells and was 1-3% of the dimers estimated to be present in the parent DNA. After 4 Jm to the -2, 0.2, 1.2, or 1.3 dimers/10 to the 9d of DNA were detected in the newly synthesized DNA of NF, 3T3 cells, or PBL respectively, and 1.0 dimer/10 to the 9d after 1 Jm-2 in XP. Analysis by fluorescence activated cell sorter analysis, incorporation studies, and autoradiography indicated that the cells were irradiated prior to S phase. Appreciable incorporation into parent strand DNA by repair synthesis was also excluded.